The study being looked at is for persons receiving dialysis being tested. This is a very high risk population with end stage kidney failure. I would presume that as a group they would have taken the masking, isolation and social distancing very seriously, so they should have low rates. This is the group that if contracting COVID got quite sick with a higher likelihood of death.
Why it is being used to looks at national rates of antibody conversion is befuddling. It tells us the rates of conversion of a very select population.